Bacteriophage Mu possesses an unusual recombination system which recognizes a specific attachment site on the phage chromosome and recombines it with an apparently random sequence in the host DNA. The long range objective of this laboratory is to understand, at the molecular level, the mechanism by which this recombination occurs. This objective will be pursued by 1) analysis of physical and genetic properties of lambda phages containing both ends of Mu (lambda Mu att phages), 2) isolation and characterization of mutants of the lambda Mu att phages defective in replication and integration, 3) determination of the block to Mu development caused by him mutations in the host and alterations in the nu phages able to overcome these blocks, and 4) isolation and characterization of other host mutants affecting Mu integration and development.